The candidate is a hematopathologist who has completed doctoral and limited postdoctoral research training. He is now entering a transitional phase in his career for which the immediate objective is to become an independent investigator as a physician-scientist. Therefore, the overall goal of this proposal is to significantly broaden and strengthen the candidate's research training thereby greatly improving his potential for success as an independent investigator in the field of hematopoiesis. At St. Jude Children's Research Hospital, the expertise and resources are readily available for the proposed development of murine models, analysis of hematopoietic cell function, and gene expression profiling analysis. The mentor, Dr. James Downing, is an acknowledged expert in hematopoiesis, molecular leukemogenesis and gene expression profiling, and his laboratory has substantial expertise in the in vitro and in vivo analysis of hematopoietic cells. Thus, St. Jude's in general and Dr. Downing's laboratory in particular provide an ideal environment in which to pursue the proposed research program. The long-term goal of this research is to investigate the mechanisms by which the AML1/CBFbeta transcription complex regulates hematopoietic cell formation and differentiation. Although this complex is essential for the induction of definitive hematopoiesis during development, the signaling cascade that is initiated in hematopoietic cells by AML1/CBFbeta remains poorly characterized. In Specific Aim 1, we will analyze the early cellular and molecular events involved in the commitment to definitive hematopoietic stem cell (HSC) formation in the developing embryo using novel murine strains that we have generated. In Specific Aim 2, we will investigate how AML1 regulates megakaryocyte (MK) cell fate commitment and development, and we will identify the AML1-dependent gene expression signature in these cells using a novel in vitro system. Under the guidance of his mentor, Dr. Downing, the candidate will develop the research expertise necessary to compete as a successful independent investigator while contributing significantly to our understanding of the role of the AML1/CBFa transcription complex in the regulation of hematopoiesis.